New Store mechanism to use with Google App Engine

Creates a Store mechanism to use with sessions when developing to Google App Engine. This is based on their own data storage system. Other methods based on file storage don't work because GAE doesn't allow file writing.
Credits to yejun (https://github.com/yejun)

An example code is:
```python
session = web.session.Session(app, web.session.GoogleStore())
```